One type of nonvolatile memory relies on a magnetoresistive effect referred to as the giant magnetoresistive (GMR) effect. GMR-based magnetic memory cells are multilayered structures comprising a nonmagnetic layer sandwiched by conductive magnetic layers. The magnetic state of the cell is determined by the relative orientation of a magnetic vector in one magnetic layer to a magnetic vector in another magnetic layer (e.g., parallel or anti-parallel). The resistance of the cell differs according to the relative orientations of the magnetic vectors. Accordingly the state of the cell can be determined by applying a voltage across the cell and measuring a resulting sense current.
The layers of magnetic material are typically formed as geometrically patterned films such as squares or rectangles. One disadvantage of patterned magnetic layer storage structures is that multiple magnetic domains may form in the magnetic layers, rendering the state of the cell indeterminate during read operations. Variation in the magnetic domain configuration of the cell may also lead to fluctuations in the magnetic switching field (i.e., coercivity) of the cell.
In order to reduce the ambiguities introduced by the magnetic domains, shape anisotropy is frequently introduced by increasing one dimension (e.g., length) of a layer with respect to another dimension (e.g., width) of the layer in order to reduce the number of domain states. One disadvantage of the rectangular shape as opposed to the square shape, however, is a significant reduction in the memory density.